Although the risk remains small, the odds are especially high
during the first trimester compared to women who become pregnant
naturally, the researchers said.

Blood clots -- called venous thromboembolism -- can develop in
the leg veins and break free, traveling to the lungs and blocking a
main artery. This condition, called pulmonary embolism, can cause
difficulty breathing and even death.

"There is an increased incidence of pulmonary embolism and venous thrombosis among women pregnant after IVF," said lead researcher Dr. Peter Henriksson, a professor of internal medicine at the Karolinska Institute in Stockholm. "Embolism is the leading cause of maternal mortality during pregnancy. The diagnosis can be elusive, so physicians should be aware of this risk to facilitate the diagnosis."

The risk of clotting during pregnancy isn't confined to women
who undergo IVF, another experts said.

"Any pregnancy carries a risk of clotting," said Dr. Avner Hershlag, chief of the Center for Human Reproduction at North Shore University Hospital in Manhasset, N.Y.

This is because hormones, particularly estrogen, increase during
pregnancy, Hershlag said. "This changes what we call the clotting
cascade," he said. "There are many factors in blood clotting that
can be affected by hormones -- especially estrogen."

In addition, the enlarging uterus puts pressure on pelvic blood
vessels, which can lead to clotting. Some women are advised to
limit their movement to reduce the risk of clotting, Hershlag
noted.

Although it's unclear why women who undergo IVF have a greater
risk of clotting, Hershlag speculates that it could be due to
fertility treatments that increase estrogen even beyond levels
normally associated with pregnancy.

The genetics of women who need IVF to conceive may also be a
factor, he added.

Worldwide, about 10 percent of couples experience infertility,
according to background information in the study. In IVF, eggs are
removed from a woman's body, fertilized by the man's sperm and
returned to her body. Since the world's first "test tube" baby was
born in 1978, about 5 million births have occurred after IVF.

For the study, published Jan. 15 in the online edition of the
journal
BMJ, researchers compared data on more than 23,000 women who
became pregnant after IVF with nearly 117,000 women who conceived
without assisted technology.

The researchers found that for women who had undergone IVF, the
risk for a blood clot was 4.2 in 1,000 women. For the women with
normal pregnancies, the risk was 2.5 in 1,000. Moreover, the risk
was highest during the first trimester.

Pulmonary embolism occurred in 19 women who had IVF (8.1 out of
10,000) compared with 70 women who conceived normally (6.0 out of
10,000), the study found.

The researchers caution that the absolute risk of a pulmonary
embolism among women who had IVF was still slight -- two to three
additional cases in 10,000 women.

"We don't want patients to get scared," Hershlag said. "Even with IVF, getting a blood clot is still pretty rare."

Also, most blood clots do not go to the lungs and can easily be
treated and resolved, Hershlag said. Women at risk for clots can be
treated with blood thinners that prevent clots, he said.

Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.